The Effects of Oxygen Concentration on Benthic Foraminiferal Growth and Size

Many organisms use oxygen through cellular respiration in order to gain energy. For this reason, oxygen has a significant influence on organism size and growth. The amount of oxygen an organism needs depends on its metabolic demand, which is partially a function organism size (i.e., mass). The Santa Monica Basin (SMB) is an oxygen minimum zone located off the southern coast of California that maintains a steep oxygen gradient and is thus an ideal location for conducting research on how oxygen influences organism size. Here we use benthic foraminifera, widespread single-celled protists that produce shells (tests), to study the controls of oxygen on organism size. Because cell mass and cell volume are correlated, we study trends in the log test volume of four abundant species from SMB: Uvigerina peregrina, Bolivina spissa, B. argentea, Loxostomum pseudobeyrichi. These foraminifera make multi-chambered tests, thus we also count the number of chambers per specimen in order to further assess their growth under varying oxygen concentrations. We analyzed the data using quantile regressions to determine trends in not only median values of the log test volume and number of chambers as a function of oxygen concentrations, but also in the 10th, 25th, 75th, and 90th percentiles because oxygen availability often constrains the maximum and minimum size of organisms. Our results show a positive correlation between oxygen concentration and the maximum log test volumes of L. pseudobeyrichi and B. argentea, supporting our hypothesis. However, we observed a negative correlation between oxygen concentration and the maximum percentiles of log test volume in U. peregrina. Nevertheless, U. peregrina still displays a positive correlation between chamber number and oxygen concentrations in line with our hypothesis. The preponderance of trends supporting a direct correlation between log test volume or chamber number and oxygen concentration suggest that oxygen limits the maximum obtainable size of benthic foraminifera through its effects on test volume or chamber growth. This study is important because it holds a glimpse into how changes in oxygen levels can affect organisms given current fluctuations in oxygen level around the world due to man-made climate change.